1. Field of the Invention
The present invention relates to an optical system for a laser beam scanner of a laser printer, and more particularly to an optical system for a laser beam scanner which is capable of reducing the manufacturing cost by use of a holographic optical element in a compensating lens array.
2. Description of the Prior Art
A conventional optical system for use in a laser beam scanner of a laser printer is illustrated in FIG. 1. As shown in FIG. 1, the conventional optical system comprises a light source 1 for generating a laser beam, a collimator 2 for allowing the laser beam from the light source 1 to advance in parallel, a cylindrical lens 3 for focusing the laser beam which has passed through the collimator 2, a rotary polyhedron 4, rotatably mounted by means of a motor (not shown), for reflecting the laser beam which has been focused by the cylindrical lens 3 toward a scanning plane of a predetermined anyle, and a compensating lens array for preventing the laser beam reflected at the polyhedron 4 from moving perpendicularly with respect to the scanning plane and scanning the laser beam on a drum 7 at a uniform speed.
The compensating lens array includes a concave lens 5 and a toric lens 6.
The toric lens 6 has a toric surface, as shown in FIGS. 2A and 2B, in which the radiuses of curvature R1 and R2 at a scanning plane and the radiuses of curvature R1' and R2' at a plane perpendicular to the scanning plane have the relationships of R1.noteq.R1' and R2=R2', respectively.
In such a conventional optical system, a laser beam 8 generated from the light source 1 is collimated by the collimator 2 and focused on the cylindrical lens 3. The focused laser beam is reflected to the scanning plane depending upon the rotation of the rotary polyhedron 4 and scanned on the drum 7 through the compensating lens array 5 and 6.
At that moment, the compensating lens array 5 and 6 prevents the laser beam 8 from slightly moving in the direction perpendicular to the scanning plane, depending upon the gradient of the mirror surface of the rotary polyhedron 4 and executes a F-.theta. compensation so that the scanning speed of the laser beam can be maintained uniform when the laser beam is scanned on the drum 7. In the F-.theta. compensation, "F" is indicative of an effective focus length of the toric lens 6 and .theta. is indicative of a rotational angle of the rotary polyhedron 4.
Such a conventional optical system, however, has a disadvantage in that since a toric lens having a toric surface is required in the compensating lens array in order to prevent the laser beam from slightly moving in the direction perpendicular to the scanning plane and to maintain the scanning speed on the drum uniform, the manufacturing cost of the optical system is high.